Debugging memory issues

When encountering memory issues or a memory leak, knowing what queries and resources are consuming a significant amount of memory is helpful. Below are queries that can help debug and find which queries, databases, and tables can be optimized:

List currently running processes by peak memory usage

SELECT
    initial_query_id,
    query,
    elapsed,
    formatReadableSize(memory_usage),
    formatReadableSize(peak_memory_usage),
FROM system.processes
ORDER BY peak_memory_usage DESC
LIMIT 100;

List metrics for memory usage

SELECT
    metric, description, formatReadableSize(value) size
FROM
    system.asynchronous_metrics
WHERE
    metric like '%Cach%'
    or metric like '%Mem%'
order by
    value desc;

List tables by current memory usage

SELECT
    database,
    name,
    formatReadableSize(total_bytes)
FROM system.tables
WHERE engine IN ('Memory','Set','Join');

Output total memory used by merges

SELECT formatReadableSize(sum(memory_usage)) FROM system.merges;

Output total memory used by currently running processes

SELECT formatReadableSize(sum(memory_usage)) FROM system.processes;

Output total memory used by dictionaries

SELECT formatReadableSize(sum(bytes_allocated)) FROM system.dictionaries;

Output total memory used by primary keys

SELECT
    sumIf(data_uncompressed_bytes, part_type = 'InMemory') as memory_parts,
    formatReadableSize(sum(primary_key_bytes_in_memory)) AS primary_key_bytes_in_memory,
    formatReadableSize(sum(primary_key_bytes_in_memory_allocated)) AS primary_key_bytes_in_memory_allocated
FROM system.parts;